Object and environment tracking via shared sensor

ABSTRACT

One disclosed example provides a head-mounted device configured to control a plurality of light sources of a handheld object and acquire image data comprising a sequence of environmental tracking exposures in which the plurality of light sources are controlled to have a lower integrated intensity and handheld object tracking exposures in which the plurality of light sources are controlled to have a higher integrated intensity. The instructions are further executable to detect, via an environmental tracking exposure, one or more features of the surrounding environment, determine a pose of the head-mounted device based upon the one or more features of the surrounding environment detected, detect via a handheld object tracking exposure the plurality of light sources of the handheld object, determine a pose of the handheld object relative to the head-mounted device based upon the plurality of light sources detected, and output the pose of the handheld object.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional from U.S. Non-Provisional patentapplication Ser. No. 15/826,560, filed Nov. 29, 2017, which claimspriority to U.S. Provisional Patent Application Ser. No. 62/503,673,filed May 9, 2017, the entirety of each of which is hereby incorporatedherein by reference for all purposes.

BACKGROUND

Motion may be tracked as computing device input via data from varioussensors, such as image sensors and inertial sensors.

SUMMARY

Examples are disclosed that relate to tracking motion of a handheldobject. One disclosed example provides a head-mounted device configuredto control a plurality of light sources of a handheld object and acquireimage data comprising a sequence of environmental tracking exposures inwhich the plurality of light sources are controlled to have a lowerintegrated intensity and handheld object tracking exposures in which theplurality of light sources are controlled to have a higher integratedintensity. The instructions are further executable to detect, via anenvironmental tracking exposure, one or more features of the surroundingenvironment, determine a pose of the head-mounted device based upon theone or more features of the surrounding environment detected, detect viaa handheld object tracking exposure the plurality of light sources ofthe handheld object, determine a pose of the handheld object relative tothe head-mounted device based upon the plurality of light sourcesdetected, and output the pose of the handheld object.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Furthermore,the claimed subject matter is not limited to implementations that solveany or all disadvantages noted in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example use scenario in which poses of a wearable deviceand handheld object may be tracked as computing device input.

FIG. 2 shows an example wearable device imaging a handheld object.

FIG. 3 shows a view of an example handheld object.

FIG. 4 shows an example computing system architecture for tracking ahandheld object via a wearable device.

FIG. 5 shows an example processing system operable to derive the pose ofa handheld object and HMD with respect to real world coordinates.

FIG. 6 shows a flow diagram illustrating an example method of estimatinga pose of a handheld object.

FIG. 7 illustrates an example of processing of IMU data on a handheldobject and on a host computing device.

FIG. 8 shows an example handheld object tracking exposure.

FIG. 9 shows an example light pulse mitigation strategy.

FIG. 10 shows another example light pulse mitigation strategy.

FIG. 11 shows plots of relative flicker amplitude as a function ofdistance from a flicker source for 30 Hz and 60 Hz flicker components.

FIG. 12 schematically shows an example method of adjusting light sourcebrightness based upon visibility of a light source on a handheld objectas a function of orientation with respect to a camera.

FIG. 13 shows a flow diagram illustrating an example method of adjustinglight source brightness.

FIG. 14A-14B show an example binary search method for optical timesynchronization.

FIG. 15 shows a block diagram of an example PLL state machine foroptical time synchronization.

FIG. 16 schematically shows light pulses in an example sweep state for aPLL state machine.

FIG. 17 schematically shows light pulses in an example roughsynchronization state for a PLL state machine.

FIG. 18 shows a graph illustrating an example method for determining atiming offset in a PLL adjust state.

FIG. 19 schematically shows an example light pulses scheme for use in aPLL adjust state of a PLL state machine.

FIG. 20 shows graphs depicting an example method for calculating atiming offset in a PLL adjust state.

FIG. 21 schematically shows another example PLL state machine.

FIG. 22 schematically shows an example light pulse cycle and an exampleexposure sequence.

FIG. 23 schematically illustrates an example method of performing ofoptical time synchronization via light pulse modulation.

FIG. 24 shows a schematic depiction of an example method of performingoptical time synchronization via active offset search.

FIG. 25 shows a flowchart illustrating an example method of determininga calibration for a stereo camera arrangement.

FIG. 26 shows a flowchart illustrating an example method of determininga calibration for a handheld object.

FIG. 27 is a block diagram showing an example computing system.

DETAILED DESCRIPTION

This application discloses examples related to computing systemscomprising head-mounted display devices (HMDs) that interface withhandheld controllers and/or other handheld peripheral objects, e.g. asparts of a gaming system. To make a gaming experience more immersive,the poses of a HMD and a handheld controller may be tracked as thedevices are moved through space by a user to provide inputs to control auser interface of the HMD. Such pose tracking also may be used for othercomputing device interactions than gaming.

The disclosed examples may utilize cameras, such as a stereo cameraarrangement, on the HMD for tracking a handheld object, such as a gamecontroller. This avoids the use of external light sources and/or camerasplaced elsewhere in the surrounding environment for tracking of thehandheld controller, which would have to be set up in the useenvironment prior to use. FIG. 1 shows an example scenario 100 in whicha HMD 102, worn by a user 104, displays virtual and/or augmented realityimagery. One or more outward-facing cameras on the HMD 102 may acquireimage data (e.g. visible light image data) of the surroundingenvironment and of a handheld object 106 held by the user 102. Forexample, FIG. 2 shows an example HMD 200 imaging a handheld object 202using a stereo camera imaging system (indicated by first camera 204 andsecond camera 206).

The term “handheld object” is used herein for an object comprisingtrackable light sources 108 that may be held by hand as describedherein, and signifies that the object may be handheld, whether or notthe object is currently in the state of being held in a hand.

The handheld object 106 is depicted as a controller, for example, for avideo game system. The handheld object 106 may, in some examples,include an onboard processor, storage system, and communication system.In other examples, the handheld object may not include one or more suchsystems, but may include lights to assist in optical tracking of thehandheld object. The handheld object 106 may also include one or moreinput controls, such as a button, trigger, joystick, directional pad,touch screen, etc. The handheld object 106 comprises an inertialmeasurement unit (IMU) (which may include an accelerometer, gyroscope,magnetometer, and/or other suitable sensors) that provides outputrelated to changes in position and orientation of the handheld object106. In some examples, the HMD 102 may also include an IMU to help trackchanges in the HMD pose in the environment.

The handheld object also includes a plurality of light sourcesdistributed over a surface of the handheld object. FIG. 3 shows anexample handheld object 300 having a plurality of light sources 302distributed along both an exterior and interior of a ring-like structureof the handheld controller 300. The light sources are configured to formpatterns of light (“constellations”) in image data acquired by thecamera or cameras of the HMD, such that a pose of the handheld objectmay be determined from an image capturing the controller. The lightsources may take any suitable form, such as light-emitting diodes (LEDs)that emit visible light for detection via a visible light camera orcameras on the HMD. Infrared light sources also may be used. In someexamples, the camera(s) on the HMD may be configured to filterwavelengths of light other than those emitted by the LEDs to reducenoise levels in the image data. A handheld object may have any suitablenumber and arrangement of light sources.

In some examples, the HMD 102 and handheld object 106 are configured tocommunicate with one another directly, such as via a wireless networkconnecting utilizing a Bluetooth communication protocol or othersuitable wireless communication protocol. In other examples, an optionalhost computing device 110 may communicate with the HMD 102 and thehandheld object 106 to receive data from HMD 102 and handheld object 106(including image data acquired by the HMD 102 and IMU data from thehandheld object 106), to process such data, and to send control signalsto these devices. FIG. 4 shows an example computing system architecture400 including an HMD 402 with tracking sensors, a host PC 404, a lefthandheld controller 406 and a right handheld controller 408, whereineach controller provides six degrees of freedom. Each of the lefthandheld controller 406 and the right handheld controller 408 isdepicted as communicating with the host PC 404 via USB (Universal SerialBus) and Bluetooth. In this example, different protocols may be used fordifferent contexts. For example, the USB connection may be used forfactory calibration while the Bluetooth may be used during ordinarydevice use. Similarly, the host PC 404 may communicate with the HMD 402and/or the handheld controllers 406, 408 via any suitable wireless orwired communication protocol. Examples are illustrated as HDMI (HighDefinition Multimedia Interface) and USB. As mentioned, host PC 404 isoptional, and each of the handheld controllers 406 and 408 maycommunicate directly with the HMD 402 in some examples. It will beunderstood that while disclosed herein in the context of handheldcontrollers, any suitable handheld or movable object may be tracked,including objects without any internal computing system or IMU, wheresuch objects include a plurality of light sources detectable as aconstellation in image data. Likewise, the specific communicationchannels shown in FIG. 4 are presented for the purpose of example, andany other suitable communications channels, wired and/or wireless, maybe used for HMD-to-host communication, host-to-handheld controllercommunication, and/or HMD-to-controller communication.

A relative-to-HMD pose (position and orientation) of the handheld object106 may be estimated by tracking the positions of light from the lightsources on the handheld object 106 using the one or more cameras on theHMD 102. Data from the IMU on the handheld object 106 can further informtracking, such as when the light sources might be occluded from view.However, the image data provides a pose of the handheld object 106relative to the HMD 102, while the IMU senses the forces proportional tothe change in position and orientation in its body frame relative to theinertial frame.

Thus, to combine the optical pose from image data with the IMU data toprovide the actual pose of the handheld object 106 relative to theworld, processing may be used to “fuse” the IMU data with the opticaldata by mapping the optical data to world coordinates. FIG. 5 shows ablock diagram of an example processing system 500 that may be used toderive the pose of the handheld object and HMD with respect to realworld coordinates from IMU data from the handheld object, and imageand/or IMU data from the HMD. The processing system 500 may beimplemented via any suitable hardware, including but not limited to theHMD, the handheld object, a host computing device in communication withthe HMD and/or the handheld object, or distributed among multiple ofthese devices and/or other devices.

In the depicted system 500, the HMD receives image data capturing thesurrounding environment and the handheld object(s) in the scene via animage sensor located on the HMD. Image data used to track thesurrounding environment is referred to herein as “environmental trackingexposures” and image data used to track handheld object(s) is referredto herein as “handheld object tracking exposures.” Various examplemethods of acquiring these exposures are described in more detail below.

An optical tracker 502 of the HMD analyzes the image data and outputsdata regarding the pose of the handheld object (or each handheld object,where multiple handheld objects are detected) with respect to the HMD(“HMD

LED Vision Poses” 503) by visually tracking the position and orientationof the handheld object per frame using the handheld object trackingexposures. Light sources may be identified in the image data, forexample, by thresholding the image to find rough light source locations,and then fitting a statistical curve (e.g. a Gaussian function) to thethresholded images to locate the light source on a pixel or sub-pixelbasis. Once the light sources in the image data are located, anysuitable method may be used to determine the position and orientation ofthe handheld object, including but not limited to epipolar line fittingand rigid body transformation fitting based on the determined lightsource locations.

A head tracker system 504 provides a pose of the HMD relative to theworld (“World

HMD Poses” at 505) by visually tracking the HMD relative to thebackground environment using the environmental tracking exposures. Anysuitable method may be used to perform the environmental featuretracking. For example, environmental features may be tracked byperforming image recognition on the environmental tracking exposures,e.g. to identify features such as edges in the image and then trackingchanges in location of the features between environmental exposureframes.

The optical pose of the handheld object with respect to the HMD (“HMD

LED Vision Poses” at 503) and the optical pose of the HMD relative tothe world (“World

HMD Poses” at 505) are used to determine a pose of the handheld objectwith respect to the world. The pose thus determined is also referred toas a ‘visual pose” herein. This pose may be provided in the form of datarepresenting changes in pose compared to a previous determination, asindicated at 506 (“World

LED Vision Changes”), or in any other suitable form. This data isadjusted via extrinsic calibration data (illustrated as “LED

IMU Extrinsics” at 508) regarding the light sources and IMU of thehandheld object to produce “World

IMU Vision Poses” at 510.

Optical pose data regarding the pose of the handheld object (“HMD

LED Vision Poses” at 503) and the pose of the HMD (“World

HMD Poses” at 505), both visually observed from the perspective of theHMD, are further provided to a fuser 514. The fuser 514 takes theoptical pose data and “fuses” it with IMU data received the handheldobject to provide an actual pose of the handheld object with respect tothe environment.

IMU data is received from IMU sources 516 on the handheld object. TheIMU data may represent six degrees of freedom. Because accelerometersand gyroscopes may have random errors which have a flat frequencyresponse that spans the whole bandwidth, a low pass filter (LPF) at 518may be used to reduce such noise by removing the high frequencycomponents that may not be used for the operation. Filtering may beperformed remotely (e.g. via a host PC) or locally, and the data alsomay be downsampled. The resulting IMU data then may be adjusted viaintrinsic calibration data 520 (illustrated as calibration source 522)previously stored for the handheld object. Example intrinsic calibrationdata 520 includes data regarding stereo camera offset and relativecamera positions. Such calibration data may be updated during systemuse, as described in more detail below. The resulting “Idealized IMU”data 524 is provided to the fuser 514 for fusing with the image datafrom the head tracker system and the optical tracker.

The handheld object and the HMD operate in separate clock domains, andcommunications channels between them may experience lag and potentiallyinconsistent data rates. As such, to help relate the IMU data to atemporally corresponding visual tracking pose 510, a time domainconverter 527 is used to align the time domains of the handheld objectwith that of the HMD. Examples of methods for performing this alignmentare described in more detail below.

Time information 525 regarding when optical poses were measured isprovided to the head tracker system 504. The head tracker system 504 maygive a most probable World

HMD pose at the time of when a corresponding HMD

LED vision pose is obtained so that the discrete time moments regardingwhen HMD

LED Vision Poses 503 and World

HMD Poses 505 are evaluated are sufficiently coincident.

As the visual pose data for the handheld object that is input to thefuser is relative to the external environment, and as the IMU data fromthe handheld object is in its body frame relative to the inertial frame,the fuser 514 may fuse these data and output the handheld object posewith respect to the world, shown as “World

LED Poses” at 526. The fuser 514 may fuse these data in any suitablemanner, such as via the use of a filter. In some examples, multipleKalman filters may be used, each operating on discrete data at discretetime stamps, to help accommodate uncertainties with regard tocorrespondence in timing of the IMU data and the visual pose data.

A “Map Changes” process 512 may be utilized to track changes in headpose with respect to different tracking features within the world, andadjust mapping when the tracking features change (e.g. when a user movesbetween rooms). “Map Changes” may be performed by rebasing worldcoordinates on a new world coordinate system, or by mapping the newworld coordinates back to the original world coordinate system. Suchupdates to the map may ensure that ongoing changes in the location ofthe HMD relative to the world are continually taken into account by thefuser 514.

The “World

HMD Poses” 526 output by the fuser may be used to derive a relation ofthe HMD to the handheld object, shown as “HMD

LED Poses” 530. “HMD

LED Poses” data 530 then may be resulting from the fuser 514 may also besent as feedback to the optical tracker 502 for motion predictionpurposes to help predict an expected pose of the controller in a nextframe based upon HMD motion and the handheld object pose. Pose datafurther may be published at 532 and sent to a pose manager 534.

FIG. 6 shows a flow diagram illustrating an example method 600 ofestimating a pose of a handheld object. Method 600 may be performed, forexample, by the optical tracker of an HMD, such as optical tracker 502of FIG. 5, and may be performed separately for each camera of a stereocamera pair. Method 600 initially includes performing, at 602, lightsource detection and sub-pixel refinement. As mentioned above, lightsources on the handheld object may be detected in handheld objecttracking exposures of image data obtained from a camera. In eachhandheld object tracking exposure, multiple light sources may bedetected. Each light source spot may cover multiple pixels. Therelatively large size of the light spot may introduce uncertainty intothe pose estimation. As such, a sub-pixel refinement may be performed todetermine a most likely sub-pixel location for each light source in theimage.

At 604, for each light source detected, a pixel location in the imagefor that light source is determined, represented by pixel coordinatesu,v. Next, at 606, each two-dimensional u,v pixel may be mapped to acorresponding candidate three-dimensional data point x, y, z based on athree-dimensional coordinate frame system in space. At 608, thecomputing device may generate a hypothesis for the pose of the handheldobject based on these light source correspondences. At 610, thecomputing device may determine four of the closest detections to the u,vpixel coordinates in pixel space and four of the closest model points tox, y, z in 3D model space. For unique combinations of 2D-3Dcorrespondences of size 4 (referring to the four closest detections andmodel points), which includes the initial u,v and x, y, z coordinates(at 612), the computing device may permute through all possibleorderings of the u,v's to fine correspondence with x, y, z's, at 614.These possible orderings are then filtered, at 616, and further verifiedthat u,v's and x, y, z's will have the same orientation once the x, y,z's are projected onto the image, at 618. At 620, the computing devicesolves for a 4-point pose, which refers to the pose (translation androtation) obtained by evaluating a correspondence of size 4. Thecomputing device further predicts more possible uv-xyz correspondences,and refines these predictions. The predictions are thresholded on areprojection error, the error between 2D detection points (u,v's) andthe prediction of associated 3D points (x, y, z), assuming a certainpose. At 622, the solution is evaluated to determine a likelihood ofthat solved pose as sufficiently matching the observed data. If the poseis determined to be sufficient, the computing device further refines thepose with multi-camera image data and returns the final pose as thesolution, at 624. For example, because the HMD may have more than onecamera, a pose may be solved for each camera on the HMD. Such refinementis performed as postprocessing of a multitude of poses obtained fromdifferent cameras. In other examples, any other suitable pose estimationprocess may be used.

FIG. 7 shows a flow diagram illustrating example processing of IMU dataon the handheld object 702 and on another computing device 704 to whichthe IMU data is sent, such as a host PC or HMD. IMU source 718 is anexample of IMU sources 516 of FIG. 5. Raw IMU data 706 from the handheldobject 702 is filtered using a low pass filter 708, as described above,and downsampled for sending to the other computing device 704. Thelow-pass-filtered IMU data 710 is received at a handheld object humaninterface device (HID) 712 of the host computing device 704 and isfurther processed by a recorder 714 and handheld object core 716 toproduce the data provided as IMU source 718.

As described above, the handheld object may use visible light LEDs toprovide pose estimation using one or more visible light cameras on theHMD. In one example, a handheld object may have an arrangement ofthirty-two visible light LEDs that may be actively illuminated fordevice tracking. Further, in some examples, two handheld objects may beutilized. In such examples, the light source arrangements for the twohandheld objects may be the same, or may be different and non-symmetricto facilitate distinguishing the light patterns arising from eachcontroller when both are visible in an image frame. The use of visiblelight sources and visible light cameras may help to avoid interferencewith foreign devices that may use infrared light, for example. Further,the use of visible light LEDs may allow the same visible light sensorsused to image the surrounding environment for tracking the HMD pose tobe leveraged for tracking the LED constellations.

As mentioned above, image data from the HMD may be used to track both apose of the handheld object relative to the HMD and a pose of the HMDrelative to the surrounding environment. However, different image datacharacteristics may be advantageous for these functions. For example,HMD pose is estimated using inside-out visible light camera sensors ofthe HMD (e.g. arranged as a stereo camera system) that track featuresdetected in images of the environment, which may image the environmentvia ambient light. Thus, to acquire sufficiently detailed images for HMDpose tracking, it may be helpful to use relatively higher camera gainand longer exposure settings (on the order of milliseconds in someexamples) to obtain images from which sufficient environmental featuresmay be extracted.

On the other hand, the handheld object pose is determined by trackinglight output by the plurality of light sources on the handheld object.As the light sources may have a relatively high intensity compared toambient light used for the HMD tracking images, a shorter cameraexposure (on the order of tens to hundreds of microseconds in someexamples) may be used to detect the light sources against the backgroundenvironment. In such images, the light sources may be detected as brightspots against a mostly black background in the shorter exposure image.

In view of these factors, acquiring head pose and handheld object poseestimation data using the same optical settings (e.g. integration time,gain) may result in either capturing sufficient background features ofthe environment but not enough contrast in the light source region, orcapturing the light source region with acceptable contrast but withinsufficient detail in the background region of the images. One possiblesolution may be to use different, dedicated cameras (or differentarrangements of plural cameras) on the HMD for each tracking function,wherein one camera or arrangement of cameras takes shorter exposure,lower gain images for light source tracking, while the other camera orarrangement of cameras takes longer exposure, higher gain images forenvironment tracking. However, the use of such dedicated cameraarrangements for environment and handheld object imaging may increasesystem cost, power consumption, and heat output.

Thus, in some examples, a same image sensing system (e.g. a camera orstereo camera arrangement) may be used for both tracking functions byseparating data from the image sensing system into separate virtualcamera streams. For example, the image sensing system may be configuredto interleave long exposure frames for environment tracking and shortexposure frames for handheld object tracking to form exposure sequences,wherein an exposure sequence is a repeating pattern of environmentaltracking exposures and handheld object tracking exposures. In someexamples, the image sensing system also may be used to acquire a greaternumber of frames for handheld object tracking than for head tracking inan exposure sequence. For example, because the handheld object maychange pose at a faster rate than the HMD, acquiring a greater number offrames for handheld object tracking than for head tracking may help tomaintain tracking fidelity between image observations.

In some examples, acquiring the environmental tracking exposurescomprises utilizing one or more of a longer exposure time and a highergain for the handheld object tracking exposures. In one more specificexample, a 120 Hz frame frequency camera may be used as a shared camerasensor in the HMD. In such a system, one example exposure sequence maycomprise a 1:1 ratio of long and short exposure times in which one 60 Hzlong (background) exposure is followed by a 60 Hz short (handheld objectlight source) exposure. Another example exposure sequence comprises a1:2 ratio of a 30 Hz long exposure followed by two 60 Hz shortexposures. As yet other example sequences, a 1:3 ratio of one longexposure followed by three short exposures may be used, or a 2:3 ratio(e.g. one long exposure, followed by one short exposure, followed by onelong exposure, followed by two short exposures) may be used. In yetother examples, any other suitable exposure sequences may be used.

FIG. 8 shows an example image 800 generated by the shared camera sensorduring a short exposure for handheld object tracking, wherein aconstellation of light sources of the handheld object is detected asilluminated spots 802 against a darkened background 804. Any suitablemethod of feature detection may be used to detect the constellation oflight sources of the handheld object. As one example, the image 800 isfirst thresholded to locate bright spots which may arise from aplurality of light sources of the handheld object. A distribution thenmay be fit (a Gaussian or other suitable distribution function) tolocate a pixel center of each candidate bright spot. Such fitting alsomay be used to reject candidate bright spots that do not match a profileexpected for the plurality of light sources of the handheld object.Based upon a location of each light source detected in the image 800, apose of the handheld object may be identified, as described in moredetail below.

In some examples, the light sources associated with the handheld objectmay be powered continuously when acquiring both environmental andhandheld object tracking exposures. However, in such examples, the lightemitted by the plurality of light sources of the handheld object mayinterfere with the camera frames corresponding to tracking a pose of theHMD (e.g., environmental tracking exposures), and also may result inhigh power consumption.

Thus, in other examples, the plurality of light sources on the handheldobject may be selectively controlled to pulse at a higher integratedintensity during the exposures corresponding to handheld objecttracking, and controlled to pulse at a lower integrated intensity duringthe exposures corresponding to environmental tracking. In some examples,the light sources may be controlled to pulse on during handheld objecttracking exposures and to pulse off during environmental trackingexposures. In either example, pulsing the light sources may help toimprove quality of the images of the background environment for HMD posetracking, reduce perceived user brightness of the handheld object lightsources, and extend battery life.

However, when light sources of the handheld object are pulsed at lowerpulse frequencies, a noticeable flicker may be perceived. As such, whereenvironmental tracking exposures and/or handheld object trackingexposures are acquired at a frame rate(s) at which flicker may benoticeable, various strategies for reducing the perceptibility of suchpulse modulation may be employed. Such strategies may be determinedbased on such factors as light pulse frequency, light intensity, andquality of the environment and handheld object images acquired by theimaging sensing system. For example, regarding frequency, light sourcepulsing may be perceptible by the human eye when the pulse frequency islower than the refresh speed of the eye. Thus, using a light pulsefrequency of 90 Hz or higher, for example, may help to reduceperceptibility of the light source modulation.

FIG. 9 shows a plot illustrating light pulses 902 and image acquisitionexposures 904, 906 as a function of time in one example pulse mitigationstrategy. In this example, the plurality of light sources of thehandheld object are pulsed for three milliseconds at a uniform 90 Hzfrequency. The handheld object tracking exposures 904, 100-400microseconds in duration in this example, are aligned with the centersof two sequential light pulses 902, and a longer 33 millisecondenvironmental tracking exposure 906 is used to acquire a backgroundenvironment image. Such a pulse may appear to the human eye as light ofa uniform intensity due to the sufficiently high modulation rate.

In the example of FIG. 9, some of the environmental tracking exposureacquisition period 906 overlaps with the light source illuminationperiod 902. To avoid such overlap, shorter, brighter pulses of the lightsources may be used for the imaging the handheld object in the handheldobject tracking frames 904. Shorter, brighter light pulses may make itmore difficult to synchronize the handheld object tracking exposures 904with the light pulses 902 output by the handheld object light sources,due for example to uncertainties in the time taken for controlinstructions to travel between a controlling device (whether a hostcomputer or an HMD) and the handheld object.

Thus, to allow light pulses of sufficient width for more certaincamera/light pulse synchronization while avoiding illumination of thelight sources during an environmental tracking exposure, a light pulsesequence may utilize light pulses arranged in various patternsconfigured to have sufficiently similar overall integral intensities tomaintain a uniform perceived brightness. FIG. 10 shows a plotillustrating light pulses 1002 and image acquisition frames 1004, 1006as a function of time for another example strategy to reduceperceptibility of light pulses modulation. In this example, acombination of flat and ramped light pulses 1002 is used. As describedin more detail below, the use of ramped light pulses may facilitateclock synchronization between the image sensing system and the handheldobject.

In some examples, the handheld object may emit one or more light pulseclusters, wherein each light pulse cluster comprises an exposure lightpulse and an antiflicker light pulse. FIG. 10 shows anti-flicker lightpulses 1008 introduced in addition to the light pulses 1002 for thehandheld object tracking exposures to form light pulse clusters 1010.Each anti-flicker light pulse 1008 may comprise a duration and intensitythat results in a “center of mass” of a light pulse cluster 1010containing the anti-flicker light pulse 1008 to be positioned at adesired frequency interval, and to have a total integrated intensity(e.g. area under the curve of all pulses in that cluster) substantiallyequal to light pulses 1002 or light pulse clusters 1010 located at otherfrequency intervals. In this example, anti-flicker light pulses 1008introduced at times 1, 2, 14, and 25 milliseconds form three light pulseclusters 1010 with centers of mass at 1.5, 12.5, 23.5 milliseconds,respectively, such that the centers of mass are approximately 11milliseconds apart (e.g. light pulses at 90 Hz). The light pulseclusters 1010 in this example are configured such that the integralintensity is equal for each light pulse cluster 1010. In this example,the plurality of light sources pulse at 90 Hz, yet may appear uniformlybright over time to the human eye.

The anti-flicker pulses 1008 may have any suitable duration(s) andintensity or intensities. The use of shorter, brighter anti-flickerpulses 1008 may allow the anti-flicker pulses 1008 to be spaced in timefarther from the environmental tracking exposures 1006, and thus providemore room to accommodate uncertainties in the synchronization betweenthe clocks of the HMD and the handheld object than the use of longer,less intense anti-flicker pulses 1008.

In cases where a time synchronization between the HMD and handheldobject is less accurate and/or it is desired to utilize longeranti-flicker pulses 1008 for a higher perceived light intensity, therisk of overlap between an anti-flicker pulse 1008 and an environmentalexposure 1006 may increase. To avoid such overlap, the timing of ananti-flicker pulse 1008 may be shifted. Such time shifting of theanti-flicker pulse 1008 may degrade the anti-flicker quality to adegree, but may still be within acceptable limits to prevent perceptionof flicker. FIG. 11 shows plots of relative flicker amplitude as afunction of distance from a flicker source (e.g. the handheld object),wherein the quality of anti-flickering decreases as the distance betweenthe HMD and the handheld object increases for both 30 Hz and 60 Hzflicker components. In this example, increased distance between the HMDand the handheld object increases a time synchronization uncertainty,and light pulses surrounding the handheld object tracking exposure maybecome longer in length to compensate. Wider light pulses surroundingthe handheld object tracking exposure may thus decrease room toincorporate anti-flickering light pulses. In FIG. 11, a limit 1102 isimposed to indicate an example range wherein light pulse modulationremains unperceived for the 30 Hz flicker component, and a limit 1104 isimposed to indicate an example range wherein light pulse modulationremains unperceived for the 60 Hz flicker component.

To determine a pose of the HMD, individual light sources on the handheldobject are identified in the image data. The HMD pose relative to thehandheld object pose then may be determined using a rigid body transformof the HMD about the handheld object or other suitable methods. Todisambiguate light sources from each other in a constellation on asingle handheld object, in some examples a Perspective-n-Point (PnP)method may be used in which an acquired image may be associated withpoints in a reference three-dimensional (3D) model to transform intocorresponding two-dimensional (2D) points in the image. The PnP methodmay be used, for example, in combination with a RANSAC method,exhaustive search method, and/or other suitable method to hypothesizeassociations between the image observations and the points in the 3Dmodel. A cost function may evaluate these hypotheses and determine whichassociations are the most accurate, eventually obtaining dataassociations and the most likely pose of the handheld object based onthe data associations. In some examples, pose prediction from previouspose estimates may also be used to assist with data associations.Gravitational information derived from IMUs of the HMD and handheldobject may also be used to help reduce the complexity of the dataassociation.

In some examples, light emitted by the light sources on the handheldobject may be modulated to help further inform a search algorithm. Asexamples, the handheld object may be controlled to turn one or more ofits light sources off, to turn one or more light sources on at specifiedtime intervals, to adjust the intensity of one or more of its lightsources to a lower intensity and the remaining light sources to a higherintensity, and/or to perform any other suitable modulation(s) that mayassist with disambiguation.

In scenarios where both left hand and right hand handheld objects areused, the left hand light source constellation may be distinguished fromthe right hand light source constellation. Likewise, where handheldobjects of an adjacent user are simultaneously viewable with thehandheld object of a user, constellations arising from the handheldobject of the adjacent user may be distinguished from the handheldobject of the user.

In scenarios involving two or more handheld objects, each handheldobject may be distinguished, for example, by defining bounding boxesaround or separately clustering light source constellations from eachhandheld object at times where the multiple handheld objects are farenough apart to distinguish clusters, e.g. at least a threshold distanceapart. For example, this threshold distance may be a predetermined via aclustering algorithm in which light sources within a threshold distanceof one another may be determined to be located on a same handheldcontroller. At times where constellations arising from multiple handheldobjects would be difficult to separate by clustering, then each handheldobject may be distinguished, for example, by emitting a unique patternand/or sub-patterns of light sources to provide exact pose estimateswith unambiguous data associations. For example, left and right handheldobjects may be configured to emit constellation patterns that arenon-symmetrical, and their unique patterns may be used to discriminateleft from right. As such, suitable search algorithms may be used toefficiently search the space of unique patterns and generate highlylikely poses. Likely poses may be refined, for example, using nonlinearoptimization techniques, to create accurate pose estimates.

As examples of methods of controlling pattern emission fordisambiguation, one handheld object may be controlled or configured toemit higher intensity light than the other handheld object, one handheldobject may be controlled or configured to turn off its light sources incertain frames, and/or the left and right handheld objects may becontrolled or configured to emit light at different frame rates.Further, motion prediction may be used to help determine when such lightsource modulation may be useful (e.g. when handheld objects are close orapproaching one another), and when clustering may be used instead (whenhandheld objects are suitably far apart and/or not approaching oneanother).

Further, in some examples, light source brightness may be used as adepth cue to help disambiguate light sources across multiple handheldobjects. For example, as described in more detail below, the intensitiesof light sources on a handheld object may be varied based upon adistance of a handheld object to the HMD. As such, if a first subset oflight sources is detected as being brighter than a second subset oflight sources, this may indicate that the first subset is associatedwith a first handheld object a greater distance from the camera, and thesecond subset is associated with a second handheld object that iscloser.

Any other suitable information from the light constellation patterns maybe utilized to help with disambiguation and pose identification. Forexample, light source brightness may be modeled as a function of dutycycle, angle time, temperature, and/or position in the image, and maythen be used to reject certain poses (e.g. these light sources cannot bethis bright and be in this pose) or to identify likely poses. Lightsource brightness also may be utilized as part of an error term in agenerative model, e.g. as described above with respect to the PnPmethod.

In yet other examples, absolute orientation information or orientationrelative to the gravity vector provided by the IMU onboard the handheldobject may be used to prune candidate light source correspondencesresulting from the PnP method. For example, light sources from foreigndevices may happen to have a same constellation, but may be filtered outwhen the device orientation as indicated by the IMU does not match, oris otherwise too far, from the previous estimated pose. In someexamples, it may also be possible to determine that a light sourceconstellation is from a foreign device if those light sources cannot becontrolled by the host computing device or HMD.

Further, once certain light source constellations are recognized asbeing from foreign devices, the system may then offset a time phase ofthe HMD camera exposures in order to avoid detecting light sourceconstellations of the foreign devices. Light sources may be pulsed toalign with the camera exposures and be offset in time such that thepulses interleave with pulses from foreign devices.

Further, in some examples, a trained machine learning model may be usedto disambiguate light source constellations. For example, such a modelmay be used to disambiguate light sources from left and right handheldobjects when the handheld objects are in close proximity, and/or todirectly predict a handheld object pose.

As mentioned above, in some examples, an intensity of each light sourcemay be dynamically adjusted based on the pose of the handheld objectrelative to the HMD. For example, the farther a distance the handheldobject is from the HMD, the brighter the light sources may need to be tomaintain signal quality, as a light source captured in an image dropsquadratically with increase in distance. On the other hand, shorterdistances may cause the light source to saturate the pixel intensity ifthe light sources are too bright. Further, light source intensity in theimage also drops with an increased angle of illumination, e.g. due torotation of the handheld object relative to the HMD, as each lightsource emits light within a cone of illumination. Additionally, lensvignetting may cause a reduction in image brightness at the peripherycompared to the image center.

Accordingly, the intensities of light sources on a handheld device maybe dynamically adjusted based upon movement of the handheld devicerelative to the cameras on the HMD to help maintain suitably uniformimage response. In one example, a method of determining how to adjustthe intensity of each light source may include a first, priming stage,in which a brightness sweep is performed from a lowest intensity to ahighest intensity until the constellation pose and light sourcecorrespondences can be successfully estimated in one or more frames.Next, in a second, tracking stage, once the light source correspondencesare determined in a frame, if a pixel intensity corresponding to a lightsource is over a saturation threshold (for example, a digital number(DN) of the image being greater than 200), then the light sourcebrightness may be reduced for the corresponding light source by aselected amount, such as a set fraction amount. In contrast, if pixelintensity is determined to be below a brightness threshold (for example,less than 55 DN) and is thus too dim, the light source brightness may beincreased for corresponding light sources by a set fraction amount.Light source brightness may be controlled by applying less or morecurrent to each light source.

The image brightness response may be modeled as a function of suchfactors as light source duty cycle, angle, time/temperature, andposition in the image. Further, motion prediction data, e.g. as obtainedfrom the fuser during processing, may be used to estimate a predictedpose of the handheld object in a future frame, and to control lightsource brightness for each light source accordingly based upon amagnitude and/or direction of the predicted motion. For example,intensity may be decreased or increased more slowly if the predictedmotion of the HMD is relatively slow, while the intensity may bedecreased or increased more quickly if the predicted motion of the HMDis relatively fast.

Dynamic adjustment of light source brightness may further be utilizedfor power saving. As mentioned above, light sources may be positioned atlocations around the handheld object. This may help to ensure that aplurality of light sources are visible to the HMD camera or cameras atany time. However, since some light sources may not be visible incertain frames and thus not used for tracking, these light sources maybe dynamically dimmed and/or turned off during those frames to conservepower. Information regarding the pose of the handheld object and thepose of the HMD, for example, may be used to determine which lightsources may and may not be visible in any image frame.

In some instances, quickly changing the brightness of each light source,including turning the light sources on and off, may be undesirable, asit may be distracting to a user (in an augmented reality scenario), andalso others in the vicinity. Thus, in some examples, the light sourcesmay be gradually dimmed to reduce jarring effects. Further, thebrightness of a light source further may be controlled based on howclose a light source is to becoming visible or invisible to the camera,e.g. based on motion prediction. This may help to prevent sudden, largemagnitude changes in intensity. In some examples, IMU data may also beused to inform motion prediction.

FIG. 12 schematically shows an example method 1200 of adjusting lightsource brightness based upon visibility of a light source 1202 on ahandheld object 1204 while it changes orientation with respect to acamera 1206. In this example, it will be understood that only one lightsource is shown and the handheld object is represented by a circle forsimplicity, but that a plurality of light sources will ordinarily bepresent. At 1208, the light source 1202 may be directly facing thecamera 1206. In this orientation, a highest intensity of light isdirected toward the camera, and as such, the light source may be set toa relatively lower intensity (which may be a function of distance fromthe camera), such as 50% (in the middle of the dynamic range of thelight source). At 1210, the handheld object is rotated such that thelight source 1202 is farther away and/or at a greater angle from thecamera 1206. As the line of sight between the light source and thecamera now is at a lower intensity region of the angular intensitydistribution of the light source, the intensity may be increased to 75%.At 1212, the light source 902 has rotated to a position that is stillvisible to the camera 1206, but almost out of sight. Thus, light at thisangle is emitted at an even lower intensity. Thus, the output may beincreased to 100% of the dynamic range. At 1214, the light source 1202has begun to rotate out of sight, and such, the brightness is decreased,e.g. back to 50% intensity. As the handheld object 1204 rotates fartherand the light source 1202 becomes completely out of view, e.g. at 180degrees opposite of the camera 1206, the light source 1202 may becompletely turned off or dimmed to a suitably low intensity.

FIG. 13 shows a flowchart illustrating an example method 1300 ofadjusting light source brightness on a handheld object. Method 1300 maybe performed, for example, by execution of instructions via a logicdevice on any suitable computing device, such as on an HMD or optionalhost PC. Method 1300 includes, at 1302, receiving image data from animage sensor (e.g. a stereo camera arrangement on an HMD), and, at 1304,detecting a set of light sources of a plurality of visible light sourceson the handheld object. Method 1300 further includes, at 1306, sending acontrol signal to the handheld device to modulate a brightness of one ormore light sources of the plurality of visible light sources. Thecontrol signal may be based on any suitable inputs. For example, thecontrol signal may be based light source characteristics detected inimage data, such as an apparent brightness of one or more light sourcesin the image data at 1308, a distance of the handheld object from theHMD at 1310 as determined from stereo image data, an orientation of thehandheld object relative to the HMD at 1312, and/or an angular locationof the handheld object in the field of view of the image sensor at 1314.

In some examples, pose prediction based on previous pose estimates maybe used to help determine when light source modulation may be useful,e.g. for disambiguation between multiple handheld objects. Poseprediction may also be used to help determine how to modulate individuallight sources on a single handheld object. Accordingly, the controlsignal may be based on motion prediction data, at 1316. For example, thelogic device may determine whether the next predicted position of alight source is farther or closer from the image sensor of the HMD. Ifthe next predicted position of the light source is farther, the lightsource may be brightened, or if the next predicted position of the lightsource is closer, then the light source may be dimmed, as examples.

The control signal may be configured to modulate brightness of the lightsources by instructing the handheld object to vary a duty cycle of powerapplied to each light source, at 1318. As discussed above, the logicdevice may instruct the handheld object to change the duty cycle at asufficient rate to hide any flicker from the human eye. In someexamples, the logic device may instruct the handheld object to apply thesame change in brightness to all light sources, at 1320. In otherexamples, the logic device may instruct the handheld object to applydifferent brightnesses to two or more light sources. Differentbrightnesses may be selected based upon any suitable factors. Forexample, different brightnesses may be based on differences in locationsof the two or more light sources on the handheld device, at 1322, suchthat the brightness of a light source that is oriented more directlytoward the image sensor is lower than a brightness of a light sourceoriented less directly toward the light source. In yet other examples, asubset of the light sources may be turned off while other light sourcesmay be illuminated, such as when the subset of light sources is orientedout of view of the image sensor, at 1324.

Due to network communication round-trip time uncertainties and otherfactors, the HMD clock domain and the handheld object clock domain maynot be synchronized at times. For example, in some systems, the HMD mayassign timestamps to data from the HMD sensors (e.g. IMU and imagedata), and the handheld object may assign timestamps to data from thehandheld object sensors (e.g. IMU data), while in other systems a hostcomputer may apply timestamps to data it receives from both the HMD andthe handheld object. In either example, time uncertainty may beintroduced by factors including data travel time over Bluetooth, datatravel time over USB, delays in data sending at the HMD, delays in datasending at the handheld object, and delays in data recognition at thehost PC, as examples. Such time uncertainty between clock domains of theHMD and the handheld object may result in environmental trackingexposures and handheld object tracking exposures being incorrectly timedrelative to the light pulses of the handheld object. This timeuncertainty also may result in incorrectly fused data. Thus, variousmethods of optical time synchronization may be used to synchronize theclock domains of the HMD and handheld object.

FIGS. 14A-14B schematically depict a first example of a method ofoptically synchronizing the clock domains of the HMD and the handheldobject. The example of FIGS. 14A-14B may be referred to herein as abinary search process 1400. A binary search process 1400 may beinitialized by determining an uncertainty interval duration, forexample, by computing the roundtrip time between a host PC and thehandheld controller over air, (e.g. Bluetooth) between a host PC and theHMD over wire (e.g. USB), or in any other suitable manner based upon aconfiguration of a system being utilized.

Once the uncertainty interval duration is understood, a binary searchprocess 1400 may be performed by first pulsing one or more light sourcesof the handheld object on for a first portion 1402 of a time interval1404, and pulsing the one or more light sources off for a second portion1406 of the time interval 1404. During this time interval 1404, the HMDimage sensing system that is shared for environmental tracking andhandheld object tracking takes a handheld object tracking exposure 1408in the time interval 1404, and determines whether the one or more lightsources are on or off in the handheld object tracking exposure 1408. Ifa light pulse 1410 of the one or more light source is not detected inthe exposure 1408, the image sensing system is determined to be out ofphase with the light source pulse frequency, and another handheld objecttracking exposure is taken in a different portion of the time interval1404. Likewise, when the handheld object tracking exposure 1408 is inthe same portion of the time interval 1404 as the light pulse 1410, theacquisition of another exposure may be omitted.

In either case, after it is determined which portion of the timeinterval 1404 contained the handheld object tracking exposure 1408, thewidth of the light pulse 1410 is reduced to a first subinterval of thefirst portion, by half in some examples, and the process is repeated forthe shortened light pulse 1412. This process may be iterativelyperformed until a target light pulse width is reached. As shown in FIG.14B, each portion of the uncertainty interval may be made to overlapwith the other portion(s) of the uncertainty interval by an exposuretime to ensure that one of the uncertainty intervals illuminates theentire handheld object tracking exposure. In the example of FIG. 14B, aduration of the light pulse 1410 a is substantially half of anuncertainty interval ±3200 μs, while a duration of light pulse 1410 b issubstantially the other half of the uncertainty interval ±3200 μs. Thesetwo light pulses 1410 a-1410 b overlap in duration by the image exposuretime such that a light pulse illuminates at least one of the handheldobject tracking exposures (image 0, image 1) acquired. Upon locating thelight pulse 1410 a in image 0, the uncertainty interval, in thisexample, is split in half to ±1600 μs, and this process of reducing thelight pulse width is iteratively performed until a target light pulsewidth 1420 is achieved.

Optical time synchronization between the HMD and the handheld object mayalso be achieved with a Phase-Locked Loop (PLL) state machine. Briefly,the light pulses and image sensing may be synchronized by shifting thetiming of the light pulses and/or image acquisition based upon therelative intensities of light pulses detected in handheld objecttracking exposures acquired during a ramp-up pulse and a ramp-downpulse.

FIG. 15 shows a block diagram depicting an example of a PLL statemachine 1500 suitable for use in aligning the phases of the HMD imagesensing system exposures and the handheld object light pulses. Theexample PLL State Machine 1500 cycles through states of sweep 1502,rough synchronization 1504, PLL adjust 1506, and optionally HID Time1508, and light pulses are ramped such that one light pulse begins at afirst intensity and is increased over a selected duration of time to asecond intensity, and another light pulse decreases over time from thefirst intensity to the second intensity.

The sweep state 1502 may begin based upon any suitable trigger, such asinitialization and light sources being undetectable in handheld objecttracking exposure(s) for a threshold duration (as measured by time ornumber of frames, for example). One example of the sweep state 1502 isshown schematically in FIG. 16, where a temporal position of a lightpulse cycle 1602 over an exposure sequence 1604 may be moved forward andbackward in time in order to detect a light pulse 1606 during at leastone handheld object tracking exposure 1608 of the exposure sequence1604. In the example of FIG. 16, the exposure sequence 1604 includes oneenvironmental tracking exposure 1610 followed by two handheld objecttracking exposures 1608, and the light pulse cycle 1602 includes a firstlight pulse 1606 having an up-ramp intensity and a second light pulse1612 having a down-ramp intensity. It will be understood that, in otherexamples, any suitable exposure sequence and light pulse cycle may beused.

In some examples, light source brightness is maintained at a constantlevel during the sweep state 1502, while in other examples, the lightsource brightness varies during the sweep state 1502. A recursive searchor binary search may be implemented during the sweep state 1502 in someexamples to more quickly converge a light pulse 1606 with a handheldobject tracking exposure 1608.

The rough synchronization state 1504, an example of which is illustratedschematically in FIG. 17, may be initiated when a constellation of lightgenerated by the light source pulse is detected in one or more handheldobject tracking exposures, such as one of every two consecutive handheldobject tracking exposures in the sweep state. Once a first light pulse1702 is detectable in a first handheld object tracking exposure 1704, inone or more light pulse cycles 1706 a-1706 c of the roughsynchronization state 1504, the relative timing between the first lightpulse 1702 and a second light pulse 1708 in the light pulse cycle isadjusted to overlap the second light pulse 1708 with a second handheldobject tracking exposure 1710. In some examples, the roughsynchronization state 1504 may continue, for example, until aconstellation of light from the light pulses is detected in twoconsecutive handheld object tracking exposures, as shown in FIG. 17 bylight pulses of the light pulse cycle 1706 c overlapping consecutivehandheld object tracking exposures of the exposure sequence 1712. Oncetwo light pulses are detected in two handheld object tracking exposuresof the exposure sequence, the PLL state machine 1500 may transition tothe PLL adjust state 1506. In some examples, a constellation of lightfrom the light pulses may become undetectable in handheld objecttracking exposures, and the PLL state machine 1500 may transition to thesweep state 1502.

In some instances, a constellation of light may be undetectable in ahandheld object tracking exposure due to reduced light sourcebrightness. Thus, in some examples, a light source brightness may bevaried over time in the rough synchronization state 1504 in suchinstances, which may help detect constellations of light from the lightpulses and may limit the duration of the rough synchronization state1504.

The PLL state machine 1500 may transition to the PLL adjust state 1506when, for example, a constellation of light is detected in twoconsecutive handheld object tracking exposures. Once a lightconstellation is detected in both handheld object tracking exposures(or, more generally, in all handheld object tracking exposures, wheremore or fewer than two exposures are used per cycle), the PLL adjuststate 1506 may distinguish the handheld object tracking exposures basedupon the relative intensities of the light pulses detected in eachexposure. For example, for light pulses having an up-ramp or a down-rampintensity, the handheld object tracking exposure may appear brighter ordimmer based upon which portion of a ramped light pulse the exposureoverlaps. Once handheld object tracking exposures acquired during rampup and ramp down are distinguished, a timing offset between the lightpulses in each light pulse cycle and the handheld object trackingexposure may be determined based upon the intensity difference betweenthe ramp up image frame and the ramp down image frame. In some examples,a timing of an exposure sequence(s) and/or timing of light pulsesemitted by the handheld object may be adjusted to correct for thedetermined timing offset.

FIG. 18 shows an example method for calculating an error e in timingoffset between a handheld object tracking exposure 1802 and a center1804 of a ramped light pulse during the PLL adjust state 1506. The imagesensing system acquires a sequence of handheld object tracking exposuresin which ramped light pulses emitted by the plurality of light sourcesare detected. After acquiring a handheld object tracking exposure 1802during each of an up-ramp light pulse 1806 and a down-ramp light pulse1808, an intensity of the light sources in each image is compared. Whena center of the handheld object tracking exposure 1802 overlaps thecenter 1804 of an up-ramp light pulse 1806 and the down-ramp light pulse1808, the light source intensities may appear uniform (or within athreshold uniformity) in the images acquired, indicating that the lightpulse sequence of the handheld object and exposure sequence are properlysynchronized. Likewise, differences in intensities between these imagesindicates that the exposures and light pulses are out ofsynchronization. In the example of FIG. 18, the center 1804 of eachramped light pulse is offset from a center of the handheld objecttracking exposure 1802, and the image acquired during the up-ramp lightpulse 1806 will appear dimmer than the image acquired during thedown-ramp light pulse 1808. The difference in image intensity eindicates the error in timing offset.

A negative feedback loop may be employed to minimize the error in timingoffset, by applying corrections A related to the observed imageintensity error e. In some examples, a proportional controller may beused where the relationship between the handheld object and the HMD isΔ=K_(p)*e. Due to clock offset drift between the two devices beingsynchronized, there may be a steady state error when such a proportionalcontroller is used. In some examples, introducing an integral componentvia a proportional-integral controller, such as that given byΔ=K_(p)*e+∫(K_(i)*e)dt, may help to mitigate steady state error. Thecoefficients K_(p) and K_(i) are to be designed for a particular system,and depend on the slope of the light pulse ramp and the amount of noisepresent in the system. Sources of noise may include errors related tolight intensity estimation, hand or head movement, and ambient lighting.In other examples, any other suitable control system may be used tominimize the offset error through the feedback loop.

The PLL adjust state 1506 is used to maintain alignment until theplurality of light sources become undetectable. However, ifconstellations of light from the light pulses are no longer detected inhandheld object tracking exposures during the PLL adjust state 1506,then the PLL state machine 1500 may change to the sweep state 1502 orother suitable state for reestablishing synchronization.

Image data noise, such as that arising from ambient light and motion ofthe handheld object relative to the HMD, may affect measurements of timeuncertainty between the clock domains. To help reduce the effect ofnoise on the PLL adjust state 1506, in some examples, an absolutemeasurement of time uncertainty may be determined using two ramped lightpulses over two consecutive handheld object tracking frames. Further, insome examples, a third light pulse of constant light intensity may beused between the up-ramp pulse and the down-ramp pulse. FIG. 19schematically shows an example light pulse sequence including a lightpulse 1902 of constant intensity positioned between an up-ramp lightpulse 1904 and a down-ramp light pulse 1906. Such a constant intensitylight pulse 1902 may have a known intensity, and thus be used to correctfor ambient light and other sources of noise. The constant intensitylight pulse 1902 also may help to distinguish the up-ramp 1904 anddown-ramp 1906 light pulses based upon their known relative positionswith respect to the constant intensity light pulse 1902. In otherexamples, two or more pulses of light of constant intensity may be usedbetween ramped pulses.

FIG. 20 shows a graphical depiction of an example method to calculatetiming offset, t_(C), in the PLL adjust state 1506 using a light pulseof constant intensity between two ramped light pulses 2002 a-b overconsecutive handheld object tracking exposures 2004 a-b, wherein theconstant intensity pulse is used to normalize the intensities of thesignals received from the ramped light pulses 2002 a-b. In this example,both the time and direction of offset may be determined. An examplecalculation for determining the timing offset t_(C) is given below inequation (1). Offset calculations may be used to decrease width of lightsource pulse ramps, which may help to conserve power and reduce aperceived brightness of the visible light sources.

$\begin{matrix}{t_{C} = {{\frac{1}{2}\left( \frac{l_{0} - l_{1}}{l_{0} + l_{1}} \right)\left( {t_{1} - t_{0}} \right)} + \left( \frac{t_{0} + t_{1}}{2} \right)}} & (1)\end{matrix}$Where t_(C) is a timing offset between light pulses and handheld objecttracking exposures, l₀ is a location at which a center of a handheldobject tracking exposure intersects a first ramped light pulse, l₁ is alocation at which a center of another handheld object tracking exposureintersects a second ramped light pulse, t₀ is a time at which eachramped light pulse begins, and t₁ is a time at which each ramped lightpulse ends.

Dynamic brightness adjustment may also be used during the PLL adjuststate 1506. For dynamic brightness, for each detected light source in animaged constellation, the median amplitude is averaged over twoconsecutive handheld object tracking frames. The average amplitude thenmay be used to modify a duty cycle to target a desired light sourceamplitude.

An optional HID time state 1508 may be used in some examples todetermine whether the PLL state machine 1500 transitions to the sweepstate 1502 or the rough synchronization state 1504. This state refers tothe above-mentioned synchronizing of the HMD and the handheld objectclock domains by computing the roundtrip time between a host PC and thehandheld object, for example, over air (e.g. Bluetooth), between a hostPC and the HMD over wire (e.g. USB), or in any other suitable mannerdepending upon a system configuration. If this process provides asufficiently small upper bound on the time offset uncertainty (e.g. +/−3ms), then the HID time state 1508 may be used to skip the sweep state1502 and may instead transition to the rough synchronization state 1504.

FIG. 21 shows another example of a PLL state machine 2100. PLL statemachine 2100 cycles through a sweep state 2102, a rough synchronizationstate 2104, a PLL adjust state 2106, and an estimated synchronizationstate 2108. The sweep state, rough synchronization state, and PLL adjuststate for the example PLL state machine 2100 may operate as describedabove for the example PLL state machine 1500.

When a light pulse(s) becomes undetectable in a handheld object trackingexposure(s) during the PLL adjust state 2106, PLL state machine 2100estimates the drift between the handheld object clock domain and the HMDclock domain in the estimated synchronization state 2108 for a durationbefore reverting to the sweep state 2102. In some examples, the driftbetween clock domains may be estimated for a threshold duration of time.If a light pulse is detected in a handheld object tracking exposureduring the estimated synchronization state 2108, the PLL state machine2100 may return to the PLL adjust state 2106 to readjust the relativetiming between clock domains of the HMD and the handheld object. On theother hand, if the light sources remain undetected for the duration ofthe estimated synchronization state 2108, the PLL state machine 2100 mayreturn to the sweep state 2102 to reestablish a synchronization.

FIG. 22 shows another example light pulse cycle 2202 and exposuresequence 2204. In this example, the exposure sequence 2204 includes twoshorter handheld object tracking exposures 2206 a-b followed by a longerenvironmental tracking exposure 2208. The light pulse cycle 2202includes an up-ramp light pulse 2210 timed to illuminate a firsthandheld object tracking exposure 2206 a, a down-ramp light pulse 2212timed to illuminate a second handheld object tracking exposure 2206 b,and an optional anti-flicker light pulse 2214. The anti-flicker lightpulse 2214 may be included in light pulse cycles for one or more of thesweep state, the PLL adjust state, and the estimated synchronizationstate, for example. As shown in FIG. 22, the anti-flicker light pulse2214 may temporally overlap the environmental tracking exposure 2208 andthus may be somewhat visible in image frames corresponding to HMD posedetermination, but may be timed such that the intensity does notinterfere unsuitably with the HMD pose determination process. Theanti-flicker pulse 2214 may be omitted during the rough synchronizationstate, in some examples, when relative timing is adjusted between lightpulses in a light pulse cycle.

Another example method of synchronizing the HMD clock and/or host clockwith a handheld object clock involves modulating the light sources onthe handheld object when the handheld object is in motion (or predictedto be in motion based upon prior and/or current motion) relative to theHMD and comparing an image capturing the modulation to an expectedimage. Based upon differences between the captured image and theexpected image, such as a light pattern detected and a light patternexpected, a timing difference between the clocks may be determined.

In such a process, the HMD camera exposure is set to a period of timelonger than that used to estimate a single handheld object pose. Duringthe period of the exposure, one or more light sources on the handhelddevice are pulsed multiple times to generate a detectable pattern in thehandheld object tracking exposure. The HMD either specifies the patternto the handheld object, or otherwise is informed of the pattern by ahost computer, and thus may compare the detected pattern to an expectedpattern. In either instance, the pattern is communicated to the handheldobject in advance so that the HMD and the handheld object aresynchronized within a suitable time based on pose uncertainty withregard to the selected pattern. The pattern detection may be performedby one camera, or multiple cameras, of the HMD.

The handheld object further may perform the pattern with a single lightsource or with a group of light sources. Performing the pattern with onelight source may increase a robustness of tracking information in theframe, whereas performing the pattern with multiple light sources mayincrease a robustness of time estimates. The patterns and actions may becomposed over time in a statistical estimation framework.

FIG. 23 schematically illustrates an example of such an approach. Inthis example, the handheld object tracking exposure time of the camerasused to capture an image of the light source modulation is set to 400microseconds, which is longer than the exposure time for estimating asingle pose of the handheld object. In the light pulse pattern 2302 ofFIG. 23, a light source is pulsed three times at higher intensity(indicated by “H” in the light pulse pattern 2302), and then three timesat lower intensity (indicated by “L” in the light pulse pattern 2302)during the 400-microsecond exposure time.

Where camera(s) of the HMD and the light pulse cycles of the handheldobject are synchronized, all light pulses are visible in the image, asshown in FIG. 23 by handheld object tracking exposure 2304. This imagemay indicate that synchronization between the HMD and the handheldobject is within a desired tolerance for adequate pose determination. Onthe other hand, where one or more higher intensity light pulses are notimaged, as shown by handheld object tracking exposure 2306, then thehandheld object clock is advancing faster than the host clock (where ahost computing device is used) and/or the HMD clock. Likewise, if one ormore lower intensity light pulses are missing in the image captured,such as in handheld object tracking exposure 2308, then the handheldobject clock is advancing more slowly than the host clock and/or HMDclock, depending upon a system configuration used. Further, a quantityof missing high or low intensity points may indicate how much of anadjustment to make to establish synchronicity within the desiredtolerance.

Yet another example method of synchronizing a handheld object clock withan HMD and/or host device clock involves acquiring a plurality ofhandheld object tracking exposures in which an illumination of theplurality of light sources in each exposure is adjusted by a differentuncertainty time relative to an exposure time, and determining the timeoffset between the clock domains of the handheld object and the HMDbased upon a brightness of each handheld object tracking exposureacquired. Determining the time offset may comprise, in some examples,performing a statistical analysis of image results to determine whichuncertainty time produces a highest intensity. Such a method may bereferred to herein as an active offset search.

As one example of an active offset search, a fixed handheld objecttracking exposure may be used with an illumination pulse of similarduration. In this example, if the uncertainty of time synchronizationbetween the HMD and the handheld object is +/−50 microseconds, then anumber of images may be captured in which the illumination pulses areintentionally offset forwards or backwards in time by an amount selectedbased upon this uncertainty. A time offset that coincides with the clockdrift may be observed as a brighter image. Similarly, a time offset inthe direction opposite to clock drift may result in a dark orunobservable image.

FIG. 24 schematically shows a sequence of handheld object trackingexposures 2402 and illumination offsets 2404 used for the series ofhandheld object tracking exposures 2402 to create a statisticalestimation framework for time clock synchronization. In this example, atime synchronization of −50 microseconds is estimated in an accumulationbuffer 2406. While not shown in this figure, the handheld objecttracking exposure may be for a same time duration as the light pulsewidth, and may be centered at offset=0. The method involves performing aseries of forward and backward shifts of the light pulse timing todetermine a time offset of maximum intensity response in the resultinghandheld object tracking exposure, and this time offset may represent anoffset in which the clocks are suitably synchronized.

As mentioned above, the handheld object pose relative to the HMD posemay be determined using such methods as a rigid body transform orepipolar line fitting. However, over time, the image sensing system ofthe HMD used to acquire image data for pose estimation may experienceperturbations. As examples, HMDs may exhibit thermal variation from use,thermal variation from environmental factors, drops, and vibrations, allof which may cause mechanical and optical deformation in the device.Such deformation may make fitting detected light constellation patternsvia rigid rotation transformation, epipolar line fitting, or otherfitting model more difficult. Cameras in a multicamera system, such as astereo camera arrangement, may move with respect to one another (e.g.rotations and translations) and may also move with respect to an IMU(when present). Camera lenses also may move and distort relative to animage sensor. All such deformations and perturbations may impacttracking quality.

Thus, examples are described herein that relate to calibrating a stereocamera arrangement of a HMD during use. The described calibrationmethods may be performed during ordinary device use, for example as abackground process, and thus may not impact a user experience when beingperformed. While described in the context of calibration during ordinaryuse, it will be understood that the disclosed examples also may be usedin an initial calibration (e.g. a factory calibration).

One possible method of calibrating may involve imaging environmentalfeatures via the stereo camera arrangement, comparing the location of anenvironmental feature in corresponding left-camera and right-cameraimages, and calibrating based on an observed offset of the environmentalfeature in the images from expected relative positions of the feature.This process may be performed, for example, by determining a pluralityof feature points in each image, determining a three-dimensionallocation of each feature point in one of the images via triangulation,and then performing a search for a companion pixel in the other image bysearching along a distorted epipolar line. When cameras in the stereocamera arrangement are in calibration, companion pixels will be locatedalong epipolar lines (each feature point detected has its owncorresponding epipolar line). Likewise, when the cameras are out ofcalibration, companion pixels will be offset from the epipolar line(s)of the other camera. An optimization function such as a Kalman filter, asliding window bundle adjustment, and/or a recursive Gauss-Newton filtermay be used to optimize this fit, and thus to determine a calibrationfor the stereo camera arrangement.

Where environmental features are used to calibrate an HMD camera system,an optimization may utilize additional variables to compensate for thetrue depths of feature points in the three-dimensional scene beingunknown. These additional variables may increase computing time and adduncertainty to the optimization algorithm. Further, when locations ofthe pixels are determined via triangulation, the optimization algorithmmay not be able to adequately distinguish baseline separation changesbetween cameras in the multicamera arrangement.

Thus, when one or more handheld object(s) are detectable in a field ofview of the HMD, light sources on the handheld object may be used asfeature points for calibration. As the geometry of the handheld objectand light sources is known to the HMD, the resulting optimization may bemore constrained and thus converge more quickly. Such a calibrationmethod also may help to determine baseline separation changes for astereo camera arrangement.

FIG. 25 shows an example method 2500 for calibrating a stereo cameraarrangement on an HMD. Method 2500 may be performed on a computingdevice, such as a HMD or host computer, via the execution ofcomputer-readable instructions. At 2502, method 2500 comprises, for eachcamera in the stereo camera arrangement, receiving image data of a fieldof view of the camera. In some examples, the image data may comprise asequence of handheld object tracking exposures and environmentaltracking exposures in a 1:1 ratio. In other examples, the image data maycomprise a greater number of handheld object tracking exposures thanenvironmental tracking exposures, or a greater number of environmentaltracking exposures than handheld object tracking exposures, dependingupon whether a higher frame rate is desired for either type of exposure.

At 2504, method 2500 comprises, for each camera in the stereo cameraarrangement, detecting a plurality of light sources of a handheld objectin the image data. Any suitable feature detection method may be used todetect the plurality of light sources. As an example, an image acquiredduring a handheld object tracking exposure may be thresholded to locatecandidate bright spots in the image, and a distribution may be fit (e.g.a Gaussian) to each candidate bright spot to locate a pixel center ofeach candidate bright spot, on a pixel or sub-pixel resolution invarious examples.

Based upon the plurality of light sources detected, method 2500comprises, at 2506, for each camera, determining a pose of the handheldobject (e.g. as expressed by labeling each light source of the handheldobject with a unique identifier). Any suitable method may be used todetermine the pose. For example, as indicated at 2508, determining thepose of the handheld object may comprise fitting an observed lightpattern to a model light pattern via a rigid body transformation. As theHMD and the handheld object may each have an initial calibration, theHMD may expect to detect a model light pattern of the handheld object.As such, a statistical analysis may analyze variations between the modellight pattern and the observed light pattern, observed over multiplehandheld object tracking exposures, to determine an error in thecalibration. As another example, determining the pose of the handheldobject may comprise searching along an epipolar line, as indicated at2510. Offsets between a detected corresponding point or pixel in animage from its expected location on an epipolar line may provide anindication of error in the calibration.

Method 2500 further comprises, at 2512, calibrating the stereo cameraarrangement based upon the pose of the handheld object determined foreach camera in the stereo camera arrangement. Calibrating the stereocamera arrangement may comprise, at 2514, calibrating one or moreintrinsic camera characteristics for each camera of the stereo cameraarrangement. Example intrinsic camera characteristics include lensdistortion, focal length, and principal point. Calibrating the stereocamera arrangement further may comprise, at 2516, calibrating one ormore extrinsic camera characteristics for each camera in the stereocamera arrangement. Example extrinsic camera characteristics include arotation and a translation of one camera relative to another camera(e.g. a transform between cameras in the image sensing system).Intrinsic and extrinsic camera characteristics may be modeled asmathematical functions in an optimization algorithm. Mathematical testsmay be applied to these mathematical functions, such as in a recursiveoptimization framework or in a bundle adjustment, to determine whethereach function is in or out of calibration. This determination maycomprise using numeric functions of error in the camera observations topredict how much error is in each parameter of the intrinsic and theextrinsic camera characteristics. Error in each calibration parametermay indicate error in pose determination, and thus the HMD may computepose based upon the updated calibration parameters. In some examples, asingle exposure may be insufficient to account for all possiblecalibration parameters, so this mathematical framework may be usedwithin a recursive optimizer.

A calibration process may be performed at any suitable frequency. Insome examples, calibration may occur 6-10 times per minute, or even over10 times per minute. In other examples, calibration may be performedless frequently. Further, as the light source locations and thus theconstellation patterns detected of the handheld object are known,calibration may be performed even where cameras do not have overlappingview of the handheld object, as long as a portion of the handheld objectis at least sometimes in view of each camera.

In some examples, inertial observations may be predicted by knowledge ofthe HMD pose relative to the handheld object. Multiple observations ofhandheld object pose by one or more cameras configured with the HMD maygive estimates over time to refine interpretation of IMU data (e.g. torecalibrate the IMU). In another example, the handheld object may bestationary. In yet another example, both the HMD and the controller maybe moving, and an optimization may be performed that may jointlyoptimize handheld object poses relative to the HMD, HMD poses relativeto the stationary environment, and a set of IMU parameters.

Handheld objects also may experience perturbations over time that impacttracking quality. As examples, handheld objects may exhibit thermalvariation from use and/or environmental factors, or may experience dropsand vibrations, all of which may cause mechanical and opticaldeformations in the handheld object. Light sources on the handheldobjects may also move with respect to one another, and/or with respectto the IMU of the handheld object, due to rotations and translations.Further, lenses of the light sources may reduce HMD accuracy and reduceuse comfort. Additionally, factory processes may introduce assemblyartifacts as the tolerances for accurate assembly may be small. Asestimated calibration parameters of a handheld object shift, trackingquality may degrade, even with initial factory calibration.

As such, various methods may be implemented to recalibrate thecalibration parameters of a handheld object. Example calibrationparameters include parameters related to light source extrinsiccharacteristics (e.g. the position of each light source with respect toother light sources on the handheld object), IMU extrinsiccharacteristics (e.g. the position of the IMU with respect to a lightsource constellation), and IMU intrinsic characteristics (e.g. bias ofgyrometer, bias of accelerometer, mixing matrices, etc.).

In one example, camera observations of the handheld object may bemodeled using mathematical functions. A natural function of calibrationsmay require accurate knowledge of the light source patterns and normalvector orientations for the direction of light. Suitable tests may beperformed on these functions to determine if they may be out ofcalibration, or in calibration. Similarly, inertial observations may bepredicted based on knowledge of the handheld object pose in a stationaryworld-locked coordinate system. Multiple observations of handheld objectpose by the HMD cameras, combined with the HMD pose in the world, maygive estimates at multiple points in time that may refine how the IMUdata is interpreted, which may be used to recalibrate the IMU. HMD posein the world may be the identity transform in the case of a stationaryHMD, or computed by an HMD tracking system (inside-out or outside-in).For example, if the IMU consistently reports data that is mismatchedfrom optical observations, calibration parameters may be adjusted asneeded to correct for IMU biases and make IMU output consistent with theoptically determined pose of the handheld object.

A natural function of calibrations may require accurate knowledge of thelight source constellations and normal vector orientations for directionof light. A secondary function may be the light source field ofillumination levels. For example, knowledge of an angle of illuminationof 50 degrees may indicates an illumination level of 50%.

As the HMD makes a sequence of observations, the HMD may determine thatalternative parameters may make the observations more likely. Forexample, after a handheld object is dropped and experiences adeformation in light source positions on the handheld object, the HMDmay observe a light pattern that differs from the model light pattern,and may recompute the constellation positions and update one or morecalibration parameters for the handheld object. The tracking system onthe HMD may use new estimates of light source positions, normal,illumination fields, and IMU parameters to refine future tracking andpose estimates.

FIG. 26 illustrates an example method 2600 for determining a calibrationfor a handheld object via a HMD comprising a stereo camera arrangement.Method 2600 may be performed on a computing device via execution ofinstructions stored on the computing device such as via a processor ofthe HMD or a host computer. Method 2600 comprises, at 2602, receiving,from a first camera of the stereo camera arrangement, first image datafrom a perspective of the first camera, and at 2604, receiving, from asecond camera of the stereo camera arrangement, second image data from aperspective of the second camera. The first image data and the secondimage data may be obtained by the first and second cameras at the sametime, and may comprise any suitable exposure sequence and frame rate(s).

At 2606, method 2600 comprises detecting, in the first image data andthe second image data, a plurality of light sources of the handheldobject. Any suitable feature detection method may be used to detect theplurality of light sources, including but not limited to those describedabove (e.g. thresholding, distribution fitting, sub-pixel refinement,etc.). Method 2600 further comprises, at 2608, determining athree-dimensional position of each light source of the plurality oflight sources detected. Determining the three-dimensional position maycomprise, for example, determining a three-dimensional position of eachlight source via stereo imaging techniques and then comparing thedetermined positions to model positions. As another example, determiningthe three-dimensional position may comprise utilizing a rigid bodytransformation to fit the observed light pattern to a three-dimensionalmodel. In either case, it may be determined that the observed patterncannot be fit to the model pattern with a suitably low error, which mayindicate a need for calibration.

Accordingly, at 2610, method 2600 comprises determining a calibrationfor the handheld object based upon each three-dimensional positiondetermined. In some examples, determining the calibration comprisesutilizing one or more of a Kalman filter, a sliding window bundleadjustment, and/or a recursive Gauss-Newton filter, to update positioninformation (e.g. rotation and/or orientation information) regardingeach individual light source. In such examples, light patterns observedover multiple handheld object tracking exposures may be used to build arobust estimate for displacement experienced by the light sources of thehandheld object.

At 2612, method 2600 may comprise receiving, from an IMU of the handheldobject, a sensed orientation of the handheld object. In some examples,the sensed orientation of the handheld object as indicated by the IMUover time may be used to help determine a change in orientation, andthus a change in pose, of the handheld object. Method 2600 may comprise,at 2614, determining a calibration for the IMU based upon the sensedorientation. As discussed above, the IMU and light sources may rotateand/or translate with respect to each other. Accordingly, determiningthe calibration for the IMU may comprise, at 2616, determining acalibration for one or more of the position and the rotation of the IMU.

In some examples, determining a calibration for the stereo cameraarrangement and determining a calibration for the handheld object may beperformed simultaneously. In such examples, the optimization framework(e.g. Kalman filter and/or other algorithms mentioned above) may be usedto solve calibration parameters for both types of calibrationssimultaneously, and thus may streamline updating calibration parametersfor each device. In other examples, the calibrations may be determinedseparately. In yet other examples, the parameters may be solved togetherin an alternating manner, where variables for one calibration processimpact the other calibration process.

The methods and processes described herein may be tied to a computingsystem of one or more computing devices, such as a head-mounted deviceor a host computer. In particular, such methods and processes may beimplemented as a computer-application program or service, anapplication-programming interface (API), a library, and/or othercomputer-program product.

FIG. 27 schematically shows a non-limiting embodiment of a computingsystem 2700 that can enact one or more of the methods and processesdescribed above. Computing system 2700 is shown in simplified form.Computing system 2700 may take the form of one or more personalcomputers, server computers, tablet computers, home-entertainmentcomputers, network computing devices, gaming devices, mobile computingdevices, mobile communication devices (e.g., smart phone), and/or othercomputing devices, such as head-mounted display device 102, handheldobject 106, and host computing device 108.

Computing system 2700 includes a logic subsystem 2702 and a storagesubsystem 2704. Computing system 2700 may optionally include a displaysubsystem 2706, input subsystem 2708, communication subsystem 2710,and/or other components not shown in FIG. 27.

Logic subsystem 2702 includes one or more physical devices configured toexecute instructions. For example, logic subsystem 2702 may beconfigured to execute instructions that are part of one or moreapplications, services, programs, routines, libraries, objects,components, data structures, or other logical constructs. Suchinstructions may be implemented to perform a task, implement a datatype, transform the state of one or more components, achieve a technicaleffect, or otherwise arrive at a desired result.

The logic subsystem 2702 may include one or more processors configuredto execute software instructions. Additionally or alternatively, logicsubsystem 2702 may include one or more hardware or firmware logicmachines configured to execute hardware or firmware instructions.Processors of logic subsystem 2702 may be single-core or multi-core, andthe instructions executed thereon may be configured for sequential,parallel, and/or distributed processing. Individual components of logicsubsystem 2702 optionally may be distributed among two or more separatedevices, which may be remotely located and/or configured for coordinatedprocessing. Aspects of logic subsystem 2702 may be virtualized andexecuted by remotely accessible, networked computing devices configuredin a cloud-computing configuration.

Storage subsystem 2704 includes one or more physical devices configuredto hold instructions executable by logic subsystem 2702 to implement themethods and processes described herein. When such methods and processesare implemented, the state of storage subsystem 2704 may betransformed—e.g., to hold different data.

Storage subsystem 2704 may include removable and/or built-in devices.Storage subsystem 2704 may include optical memory (e.g., CD, DVD,HD-DVD, Blu-Ray Disc, etc.), semiconductor memory (e.g., RAM, EPROM,EEPROM, etc.), and/or magnetic memory (e.g., hard-disk drive,floppy-disk drive, tape drive, MRAM, etc.), among others. Storagesubsystem 2704 may include volatile, nonvolatile, dynamic, static,read/write, read-only, random-access, sequential-access,location-addressable, file-addressable, and/or content-addressabledevices.

It will be appreciated that storage subsystem 2704 includes one or morephysical devices. However, aspects of the instructions described hereinalternatively may be propagated by a communication medium (e.g., anelectromagnetic signal, an optical signal, etc.) that is not held by aphysical device for a finite duration.

Aspects of logic subsystem 2706 and storage subsystem 2704 may beintegrated together into one or more hardware-logic components. Suchhardware-logic components may include field-programmable gate arrays(FPGAs), program- and application-specific integrated circuits(PASIC/ASICs), program- and application-specific standard products(PSSP/ASSPs), system-on-a-chip (SOC), and complex programmable logicdevices (CPLDs), for example.

When included, display subsystem 2706 may be used to present a visualrepresentation of data held by storage subsystem 2704. This visualrepresentation may take the form of a graphical user interface (GUI). Asthe herein described methods and processes change the data held by thestorage machine, and thus transform the state of the storage machine,the state of display subsystem 2706 may likewise be transformed tovisually represent changes in the underlying data. Display subsystem2706 may include one or more display devices utilizing virtually anytype of technology. Such display devices may be combined with logicsubsystem 2702 and/or storage subsystem 2704 in a shared enclosure, orsuch display devices may be peripheral display devices.

When included, input subsystem 2708 may comprise or interface with oneor more user-input devices such as a keyboard, mouse, touch screen, orgame controller. In some embodiments, the input subsystem may compriseor interface with selected natural user input (NUI) componentry. Suchcomponentry may be integrated or peripheral, and the transduction and/orprocessing of input actions may be handled on- or off-board. Example NUIcomponentry may include a microphone for speech and/or voicerecognition; an infrared, color, stereoscopic, and/or depth camera formachine vision and/or gesture recognition; a head tracker, eye tracker,accelerometer, and/or gyroscope for motion detection and/or intentrecognition; as well as electric-field sensing componentry for assessingbrain activity.

When included, communication subsystem 2710 may be configured tocommunicatively couple computing system 2700 with one or more othercomputing devices. Communication subsystem 2710 may include wired and/orwireless communication devices compatible with one or more differentcommunication protocols. As non-limiting examples, the communicationsubsystem may be configured for communication via a wireless telephonenetwork, or a wired or wireless local- or wide-area network. In someembodiments, the communication subsystem may allow computing system 2700to send and/or receive messages to and/or from other devices via anetwork such as the Internet.

Another example provides a head-mounted device, comprising an imagesensing system, a logic device configured to execute instructions, and astorage device comprising instructions executable by the logic device tocontrol a plurality of light sources of a handheld object, acquire imagedata of a surrounding environment, the image data comprising a sequenceof environmental tracking exposures in which the plurality of lightsources are controlled to have a lower integrated intensity and handheldobject tracking exposures in which the plurality of light sources arecontrolled to have a higher integrated intensity, detect, via anenvironmental tracking exposure, one or more features of the surroundingenvironment, determine a pose of the head-mounted device based upon theone or more features of the surrounding environment detected, detect,via a handheld object tracking exposure, the plurality of light sourcesof the handheld object, determine a pose of the handheld object relativeto the head-mounted device based upon the plurality of light sourcesdetected, and output the pose of the handheld object determined. In suchan example, the image sensing system may additionally or alternativelycomprise a stereo camera arrangement. In such an example, the imagesensing system may additionally or alternatively comprise one or morevisible light camera sensors. In such an example, the instructions mayadditionally or alternatively be executable to modulate an intensity ofthe plurality of light sources at a frequency of 90 Hz or greater. Insuch an example, the instructions may additionally or alternatively beexecutable to acquire the handheld object tracking exposures at a higherframe rate than the environmental tracking exposures. In such anexample, acquiring the environmental tracking exposures may additionallyor alternatively comprise one or more of a longer exposure time and ahigher gain than acquiring the handheld object tracking exposures. Insuch an example, the sequence of environmental tracking exposures andhandheld object tracking exposures may additionally or alternativelycomprise a ratio of one environmental tracking exposure to two handheldobject tracking exposures. In such an example, acquisition of theenvironmental tracking exposure may additionally or alternativelyoverlap with an illumination of the plurality of light sources of thehandheld object. In such an example, the instructions may additionallyor alternatively be executable to output one or more light pulseclusters, each light pulse cluster comprising an exposure light pulseand an antiflicker light pulse.

Another example provides a head-mounted device, comprising an imagesensing system, a logic device configured to execute instructions, and astorage device comprising instructions executable by the logic device tocontrol a plurality of light sources of a handheld object, acquire, viathe image sensing system, image data of the surrounding environment, theimage data comprising a sequence of environmental tracking exposures andhandheld object tracking exposures, determine whether the handheldobject is detectable in one or more handheld object tracking exposures,and based upon the handheld object not being detectable in the one ormore handheld object tracking exposures, perform an optical timesynchronization between a clock domain of the head-mounted device and aclock domain of the handheld object. In such an example, determiningwhether the handheld object is detectable in the one or more handheldobject tracking exposures may additionally or alternatively comprisedetermining whether a plurality of visible light sources of the handheldobject is detectable in the one or more handheld object trackingexposures. In such an example, the instructions may additionally oralternatively be executable to perform the optical time synchronizationvia a binary search process comprising pulsing a plurality of lightsources of the handheld object on for a first portion of a time intervaland off for a second portion of the time interval, acquiring, via theimage sensing system, a first handheld object tracking exposure,determining whether the first handheld tracking exposure occurred duringthe first portion or the second portion of the time interval, based ondetermining that the first handheld tracking exposure occurred duringthe first portion of the time interval, reducing a light pulse width toa first subinterval of the first portion of the time interval,acquiring, via the image sensing system, a second handheld objecttracking exposure, determining whether the second handheld objecttracking exposure occurred during the first subinterval, and based ondetermining that the second handheld object tracking exposure occurredduring the first subinterval, reducing the light pulse width to a secondsubinterval of the first subinterval. In such an example, theinstructions may additionally or alternatively be executable to performthe optical time synchronization by acquiring a plurality of handheldobject tracking exposures in which an illumination of the plurality oflight sources in each exposure is adjusted by an uncertainty timerelative to an exposure time, and determining, based upon a brightnessof each handheld object tracking exposure acquired, a time offsetbetween the clock domain of the handheld object and the clock domain ofthe head-mounted device. In such an example, the instructions mayadditionally or alternatively be executable to perform the optical timesynchronization by, upon the handheld object being determined to be inmotion or predicted to be in motion, controlling one or more lightsource of the handheld object to pulse in a pattern, acquiring, via theimage sensing system, a first handheld object tracking exposure, andcomparing the pattern expected to a pattern detected in the firsthandheld object tracking exposure. In such an example, the instructionsmay additionally or alternatively be executable to perform the opticaltime synchronization via a phase locked loop adjust state by comparingan intensity of a first light pulse detected in a first handheld objecttracking exposure to an intensity of a second light pulse detected in asecond handheld object tracking exposure, and based upon the comparison,adjust a relative timing of the clock domain of the handheld object andthe clock domain of the head-mounted device. In such an example, theinstructions may additionally or alternatively be executable to, basedupon the handheld object being undetectable for a threshold number ofhandheld object tracking exposures, adjust one or more of an intensityof a light pulse and a timing between light pulses to overlap a firstlight pulse with a first handheld object tracking exposure and overlap asecond light pulse with a second handheld object tracking exposure priorto the phase locked loop adjust state. In such an example, theinstructions may additionally or alternatively be executable to, priorto the phase locked loop adjust state, increase the intensity of thefirst light pulse over a time duration of the first light pulse, anddecrease the intensity of the second light pulse over a time duration ofthe second light pulse.

Another example provides a head-mounted device configured to synchronizea clock domain of the head-mounted device with a clock domain of ahandheld object in communication with the head-mounted device, thehead-mounted device comprising an image sensing system, a logic deviceconfigured to execute instructions, and a storage device comprisinginstructions executable by the logic device to control a plurality oflight sources of the handheld object to emit a sequence of light pulses,the sequence comprising a plurality of light pulse cycles each includingat least a first light pulse and a second light pulse, receive, via theimage sensing system, image data comprising a sequence of handheldobject tracking exposures and environmental tracking exposures, basedupon the plurality of light sources not being detectable in the imagedata, in one or more light pulse cycles of a sweep state, shift atemporal position of the light pulse cycles one or more of forward intime and backward in time to detect a first light pulse in a firsthandheld object tracking exposure, when a first light pulse isdetectable in the first handheld object tracking exposure, in one ormore light pulse cycles of a rough synchronization state, adjust atiming between the first light pulse and the second light pulse tooverlap the second light pulse with a second handheld object trackingexposure, when the second light pulse is determined to overlap with thesecond handheld object tracking exposure, in one or more light pulsecycles of a phase locked loop state, compare an intensity of a firstlight pulse detected in the first handheld object tracking exposure toan intensity of a second light pulse detected in the second handheldobject tracking exposure, and based upon the comparison, adjust arelative timing of the clock domain of the handheld object and the clockdomain of the head-mounted device. In such an example, the instructionsmay additionally or alternatively be executable to, in the one or morelight pulse cycles of the sweep state, vary a brightness of one or morelight pulses. In such an example, the instructions may additionally oralternatively be executable to adjust the relative timing by adjustingone or more of an exposure sequence timing, a timing between lightpulses, and a timing between light pulse cycles.

It will be understood that the configurations and/or approachesdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are possible. The specific routines ormethods described herein may represent one or more of any number ofprocessing strategies. As such, various acts illustrated and/ordescribed may be performed in the sequence illustrated and/or described,in other sequences, in parallel, or omitted. Likewise, the order of theabove-described processes may be changed.

The subject matter of the present disclosure includes all novel andnon-obvious combinations and sub-combinations of the various processes,systems and configurations, and other features, functions, acts, and/orproperties disclosed herein, as well as any and all equivalents thereof.

The invention claimed is:
 1. A head-mounted device, comprising: an imagesensing system; a logic device; and a storage device comprisinginstructions executable by the logic device to control a plurality oflight sources of a handheld object; acquire, via the image sensingsystem, image data of a surrounding environment, the image datacomprising an exposure sequence, the exposure sequence comprising arepeating pattern of environmental tracking exposures and handheldobject tracking exposures; determine whether the handheld object isdetectable in one or more of the handheld object tracking exposures; andbased upon the handheld object not being detectable in the one or moreof the handheld object tracking exposures, perform an optical timesynchronization between a clock domain of the head-mounted device and aclock domain of the handheld object.
 2. The head-mounted device of claim1, wherein determining whether the handheld object is detectable in theone or more of the handheld object tracking exposures comprisesdetermining whether at least some light sources of the plurality oflight sources of the handheld object are detectable in the one or moreof the handheld object tracking exposures.
 3. The head-mounted device ofclaim 1, wherein the instructions are executable to perform the opticaltime synchronization via a binary search process comprising pulsing theplurality of light sources of the handheld object on for a first portionof a time interval and off for a second portion of the time interval,acquiring, via the image sensing system, a first handheld objecttracking exposure of the exposure sequence, determining whether thefirst handheld tracking exposure occurred during the first portion orthe second portion of the time interval, based on determining that thefirst handheld tracking exposure occurred during the first portion ofthe time interval, reducing a light pulse width to a first subintervalof the first portion of the time interval, acquiring, via the imagesensing system, a second handheld object tracking exposure of theexposure sequence, determining whether the second handheld objecttracking exposure occurred during the first subinterval, and based ondetermining that the second handheld object tracking exposure occurredduring the first subinterval, reducing the light pulse width to a secondsubinterval of the first subinterval.
 4. The head-mounted device ofclaim 1, wherein the instructions are executable to perform the opticaltime synchronization by acquiring a plurality of handheld objecttracking exposures of the exposure sequence in which an illumination ofthe plurality of light sources in each exposure is adjusted by anuncertainty time relative to an exposure time; and determining, basedupon a brightness of each handheld object tracking exposure of theplurality of handheld object tracking exposures of the exposuresequence, a time offset between the clock domain of the handheld objectand the clock domain of the head-mounted device.
 5. The head-mounteddevice of claim 1, wherein the instructions are executable to performthe optical time synchronization by, upon the handheld object beingdetermined to be in motion or predicted to be in motion, controlling oneor more light sources of the plurality of light sources of the handheldobject to pulse in a pattern; acquiring, via the image sensing system, afirst handheld object tracking exposure of the exposure sequence; andcomparing a pattern of light sources expected to a pattern of lightsources detected in the first handheld object tracking exposure.
 6. Thehead-mounted device of claim 1, wherein the instructions are executableto perform the optical time synchronization via a phase locked loopadjust state by comparing an intensity of a first light pulse detectedin a first handheld object tracking exposure acquired in the exposuresequence to an intensity of a second light pulse detected in a secondhandheld object tracking exposure acquired in the exposure sequence, andbased upon the comparison, adjust a relative timing of the clock domainof the handheld object and the clock domain of the head-mounted device.7. The head-mounted device of claim 6, wherein the instructions areexecutable to, based upon the handheld object being undetectable for athreshold number of handheld object tracking exposures, adjust one ormore of an intensity of a light pulse and a timing between light pulsesto overlap a first light pulse with the first handheld object trackingexposure and overlap a second light pulse with the second handheldobject tracking exposure prior to the phase locked loop adjust state. 8.The head-mounted device of claim 6, wherein the instructions areexecutable to, prior to the phase locked loop adjust state, increase theintensity of the first light pulse over a time duration of the firstlight pulse, and decrease the intensity of the second light pulse over atime duration of the second light pulse.
 9. A head-mounted deviceconfigured to synchronize a clock domain of the head-mounted device witha clock domain of a handheld object in communication with thehead-mounted device, the head-mounted device comprising: an imagesensing system; a logic device; and a storage device comprisinginstructions executable by the logic device to control a plurality oflight sources of the handheld object to emit a sequence of light pulses,the sequence comprising a plurality of light pulse cycles each includingat least a first light pulse and a second light pulse, receive, via theimage sensing system, image data comprising an exposure sequence, theexposure sequence comprising a repeating pattern of handheld objecttracking exposures and environmental tracking exposures, based upon theplurality of light sources not being detectable in the image data, inone or more light pulse cycles of a sweep state, shift a temporalposition of the one or more light pulse cycles of the sweep state one ormore of forward in time and backward in time to detect the first lightpulse in a first handheld object tracking exposure acquired during thesweep state; when the first light pulse is detectable in the firsthandheld object tracking exposure acquired during the sweep state, inone or more light pulse cycles of a rough synchronization state, adjusta timing between the first light pulse and the second light pulse tooverlap the second light pulse with a second handheld object trackingexposure acquired during the rough synchronization state; when thesecond light pulse is determined to overlap with the second handheldobject tracking exposure acquired during the rough synchronizationstate, in one or more light pulse cycles of a phase locked loop state,compare an intensity of a first light pulse detected in the firsthandheld object tracking exposure to an intensity of a second lightpulse detected in the second handheld object tracking exposure, andbased upon the comparison, adjust a relative timing of the clock domainof the handheld object and the clock domain of the head-mounted device.10. The head-mounted device of claim 9, wherein the instructions areexecutable to, in the one or more light pulse cycles of the sweep state,vary a brightness of one or more light pulses.
 11. The head-mounteddevice of claim 9, wherein the instructions are executable to adjust therelative timing by adjusting one or more of an exposure sequence timing,a timing between light pulses, and a timing between light pulse cycles.12. On a head-mounted device comprising an image sensing system, amethod, comprising: controlling a plurality of light sources of ahandheld object; acquiring, via the image sensing system, image data ofa surrounding environment, the image data comprising an exposuresequence, the exposure sequence comprising a repeating pattern ofenvironmental tracking exposures and handheld object tracking exposures;determining whether the handheld object is detectable in one or more ofthe handheld object tracking exposures; and based upon the handheldobject not being detectable in the one or more of the handheld objecttracking exposures, performing an optical time synchronization between aclock domain of the head-mounted device and a clock domain of thehandheld object.
 13. The method of claim 12, wherein determining whetherthe handheld object is detectable in the one or more of the handheldobject tracking exposures comprises determining whether at least somelight sources of the plurality of light sources of the handheld objectare detectable in the one or more of the handheld object trackingexposures.
 14. The method of claim 12, comprising performing the opticaltime synchronization via a binary search process comprising pulsing theplurality of light sources of the handheld object on for a first portionof a time interval and off for a second portion of the time interval,acquiring, via the image sensing system, a first handheld objecttracking exposure of the exposure sequence, determining whether thefirst handheld tracking exposure occurred during the first portion orthe second portion of the time interval, based on determining that thefirst handheld tracking exposure occurred during the first portion ofthe time interval, reducing a light pulse width to a first subintervalof the first portion of the time interval, acquiring, via the imagesensing system, a second handheld object tracking exposure of theexposure sequence, determining whether the second handheld objecttracking exposure occurred during the first subinterval, and based ondetermining that the second handheld object tracking exposure occurredduring the first subinterval, reducing the light pulse width to a secondsubinterval of the first subinterval.
 15. The method of claim 12,comprising performing the optical time synchronization by acquiring aplurality of handheld object tracking exposures of the exposure sequencein which an illumination of the plurality of light sources in eachexposure is adjusted by an uncertainty time relative to an exposuretime; and determining, based upon a brightness of each handheld objecttracking exposure of the plurality of handheld object tracking exposuresof the exposure sequence, a time offset between the clock domain of thehandheld object and the clock domain of the head-mounted device.
 16. Themethod of claim 12, comprising performing the optical timesynchronization by, upon the handheld object being determined to be inmotion or predicted to be in motion, controlling one or more lightsources of the plurality of light sources of the handheld object topulse in a pattern; acquiring, via the image sensing system, a firsthandheld object tracking exposure of the exposure sequence; andcomparing a pattern of lights sources expected to a pattern of lightsources detected in the first handheld object tracking exposure.
 17. Themethod of claim 12, comprising performing the optical timesynchronization via a phase locked loop adjust state by comparing anintensity of a first light pulse detected in a first handheld objecttracking exposure acquired in the exposure sequence to an intensity of asecond light pulse detected in a second handheld object trackingexposure acquired in the exposure sequence, and based upon thecomparison, adjust a relative timing of the clock domain of the handheldobject and the clock domain of the head-mounted device.
 18. The methodof claim 17, comprising, based upon the handheld object beingundetectable for a threshold number of handheld object trackingexposures, adjusting one or more of an intensity of a light pulse and atiming between light pulses to overlap a first light pulse with thefirst handheld object tracking exposure and overlap a second light pulsewith the second handheld object tracking exposure prior to the phaselocked loop adjust state.
 19. The method of claim 17, comprising, priorto the phase locked loop adjust state, increasing the intensity of thefirst light pulse over a time duration of the first light pulse, anddecreasing the intensity of the second light pulse over a time durationof the second light pulse.